The Border Gateway Protocol (BGP) is an interautonomous system routing protocol. An autonomous system (AS) is a network or group of networks under a common administration and with common routing policies. BGP is used to exchange routing information for the Internet and is the protocol used between Internet Service Providers (ISPs). Customer networks usually employ an Interior Gateway Protocol (IGP) such as Routing Interior Protocol (RIP), which specifies how routers exchange routing table information. With RIP, routers periodically exchange entire tables. Another IGP is known as Open Shortest Path First (OSPF) protocol. In OSPF the routers use link-state algorithms to send routing information to all nodes in an internetwork by calculating the shortest path to each node based on a topography of the network constructed by each node. Each router sends the particular portion of the routing table that describes the state of its own links, and it also sends the complete routing structure (topography). An advantage of OSPF is that it results in smaller more frequent updates everywhere. The updates converge quickly, thus preventing such problems as routing loops and Count-to-Infinity (when routers continuously increment the hop count to a particular network). This makes for a more stable network.
Customers connect to ISPs, and ISPs use BGP to exchange customer and ISP routes. When BGP is used between autonomous systems the protocol is referred to as External BGP (EBGP). If a service provider is using BGP to exchange routes within an AS, then the protocol is referred to as Interior BGP (IBGP). BGP neighbors exchange full routing information when the TCP connection between neighbors is first established. When changes to the routing table are detected, the BGP routers send to their neighbors only those routes that have changed. BGP routers do not send periodic routing updates, and BGP routing updates advertise only the optimal path to a destination network.
BGP uses many route parameters to define routing policies and maintain a stable routing environment. Routes learned via BGP have associated properties (also referred to as attributes) that are used to determine the best route to a destination when multiple paths exist to a particular destination. These attributes include: Weight, Local Preference, Multi-Exit Discriminator, Origin, AS_Path, Next Hop and Community.
BGP also has mechanisms such as Outbound Route Filtering (ORF) which enable the proper set of Virtual Private Network (VPN) routing distribution constraints to be dynamically distributed. This reduces the management burden of setting up the constraints, and results in improved scalability.
Within a single routing domain it is common to have the IBGP routers peer directly with one or two route reflectors (described below), rather than having them peer directly with each other. This greatly reduces the number of IBGP adjacencies which any one router must support. Further, a route reflector does not merely redistribute routing information, it “digests” the information first, by running its own decision processes. Only routes which survive the decision process are redistributed.
A route reflector reflects routes between the two groups: client peers and all the other routers in the autonomous system (non-client peers). The route reflector and its client peers form a cluster. The non-client peers are fully meshed with each other, but the client peers need not be fully meshed. When the route reflector receives an advertised route from an external BGP speaker, the route is advertised to all clients and non-client peers. When the route reflector receives an advertised route from a non-client peer, the route is advertised to all clients. When the route reflector receives a route from a client, the route is advertised to all clients and non-client peers. Hence, the clients need not be fully meshed.
As a result, when route reflectors are used, the amount of routing information carried around the network, and in particular, the amount of routing information which any given router must receive and process, is greatly reduced. This greatly increases the scalability of the routing distribution system.
An Automated ORF-list mechanism that provides the ability to filter routes at the Route Reflector (RR) is described in co-pending patent application No. 10/855769, filed May 27, 2004, titled AUTOMATIC ORF-LIST CREATION FOR ROUTE PARTITIONING ACROSS BGP ROUTE REFLECTORS, the disclosure of which is incorporated by reference herein. The result of the mechanisms described in the above-identified co-pending application number is the aggregation of route filters (ORF lists) at the RRs and the scaling of Provider Edge (PE) routers through filtering of unwanted route information. This filtering is done by the RRs by not advertising unwanted routes to the PEs.